Synthesis and characterization of cobalt(III) and cobalt(IV) complexes of N-(2-hydroxyphenyl)salicylamide and homologues

Abstract

Tridentate trianionic chelating ligands, N-(2-hydroxyphenyl)salicylamide (H₃L¹) and its homologues with a substituent on the 2-hydroxyphenyl moiety (5-Me, H₃L²; 5-Cl, H₃L³; 4-NO₂, H₃L⁴; 5-NO₂, H₃L⁵), were utilized for the synthesis of cobalt complexes. The ligands H₃L¹ and H₃L² gave uncharacterized substances when treated with a cobalt(II) salt, whereas H₃L³—H₃L⁵ formed cobalt(III) complexes of general formula K₃[CoL₂.] The cobalt(III) complexes were characterized by elemental analyses, thermogravimetric analyses, electronic spectra, and cyclic voltammograms. Cyclic voltammograms showed two redox couples at significantly low potential, which were assigned to the Co^IV–Co^III and Co^V–Co^IV redox processes based on controlled-potential electrolysis and e.s.r. investigations. The cobalt(III) complexes were readily oxidized chemically (with I₂,) or electrochemically to the cobalt(IV) complexes, which were identified by an e.s.r. signal near g= 2.0 with an eight-line hyperfine structure (A_av.= 0.001 92 cm^–1).